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Volcano, a Quaternary volcanic centre of the eastern Aleutian Arc, produces predominantly andesiies and dacites of low-to medium-K calc-alkaline composition. Mineralogical and major element characteristics of representative lavas suggest that magmatic evolution has been influenced by both crystal fractionation and magma-mixing processes. However, incompatible trace element variations (e.g. K/Rb) indicate that these evolved lavas have been contaminated by the mafic arc crust of the underlying Talkeetna accreted terrane. The limited range of isotope compositions also supports the assimilation of nonradiogenic mafic crust (e.g. S7 Sr/ S6 Sr = 0 7032-0-7034; m Nd/ I44 Nd = 0-51301-0-5130). In addition, Pb-isotope compositions parallel the North Pacific mean oceanic trend (2(k Pb/ 2M Pb = 18-3-18-8; 2o/ Pb/ 2O4 Pb = 15-5-15-6; 208 Pb/ 204 Pb-38-2-38-3) and do not require a subducted sediment component in the source. Relatively high (Ba/La) # (0-79-18-10) and B/Be (14-5) ratios do, however, suggest a metasomatic fluid component derived from the dehydration of the subducting plate. The thickened continental crust (~35 km) of the eastern Aleutian Arc prevents the ascent of basaltic melts, which fractionate and assimilate at various depths to produce andesitic magmas. These andesites evolve towards more silicic compositions by fractional crystallization. The absence of evidence for a large high-level crustal magma chamber implies that the magmatic system beneath the volcano is young and at an immature stage of evolution.

The East Kemptville greisen-hosted tin deposit (58 million tonnes of 0.165 percent Sn), southwestern Nova Scotia, Canada, occurs beneath undulations in the contact between granitic rocks of the Davis Lake complex (DLC) and Meguma Group metawacke. Cassiterite-topaz ore precipitated from a F-and Sn-rich fluid derived from the East Kemptville leucomonzogranite. Controls on tin mineralization include the unusual primary Sn and F abundances of the DLC magma, the vertical chemical zonation of the magma prior to crystallization, generation of an aqueous phase, and associated Sn-CI complexing before the separation of F from the magma and the flat-lying granitemetawacke contact.The Carboniferous DLC is composed of biotite-bearing monzogranites, leucomonzogranite, and high-F, low-B topaz greisen. The chemical and isotopic signatures of the monzogranites reflect the unusual source of this highly evolved pluton. Magmatic evolution was focused toward the granite-metawacke contact and culminated, at the current erosional level, in the East Kemptville deposit. Chemical variation is attributed to the vertical zonation of the magma prior to the fractionation of biotite, K-feldspar, ilmenite, and REE-bearing phosphate minerals. The remarkably high F and P contents resulted in unusual A1 and P distributions and concentration of metals beneath the contact as a result of increased magmatic depolymerization and diffusion.Pegmatitic segregations record the evolution of a F-and Sn-rich aqueous fluid. This aqueous phase scavenged CI, alkali elements, P, Sn, and other metals from the magma, concentrating them beneath the contact. In contrast to porphyry-style deposits, this fluid was not expelled from the granite. When crystallization was complete, more than 10,000 tonnes of F partitioned into this aqueous fluid, forming a hydrofluoric fluid that was neutralized by reaction with the leucomonzogranite. Quartz-topaz rock is either a direct precipitate from the F-rich fluid or extensively replaced leucomonzogranite. Quartz-mica greisen and incomplete greisen formed as the F/OH ratio in the fluid decreased. Veins and fractures emanating from massive greisen zones have alteration envelopes, indicating these structures were conduits for the F-rich fluid. Their orthogonal orientation suggests these veins are related to cooling, not hydraulic fracturing.Whole-rock Rb-Sr isotopic data from leucomonzogranite and quartz-topaz rocks yield dates and initial ratios statistically identical to the biotite monzogranite. High mean square of weighted deviations (MSWDs) resulting from the former data reflect contami-*Present address: , 1990, Genesis and fluid evolution of the East Kemptville greisen-hosted tin mine, southwestern Nova Scotia, Canada, in Stein, H. J., and Hannah, J. L., eds., Ore-bearing granite systems; Pedogenesis and mineralizing processes: Geological Society of America Special Paper 246. J. M. Richardson and Othersnation of the late-magmatic fluids by Sr derived from the metawacke and later thermal overprinting. Deformation, probably relat...

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The rheniumosmium decay scheme (187Re --t 1870s) offers a unique opportunity t o investigate the genesis of, and directly date, ultramafic rock, sulphide and platinum ore deposits. Inductively coupled plasma mass spectrometry (ICP-MS) is a viable method for 0 s isotopic analysis as it provides the high temperatures necessary t o ionise 0s. The sample can be introduced into the ICP mass spectrometer either by conventional nebulisation or by distillation using an 0 s "generator." Generator-mode analysis is superior t o nebulisation because (i) the total number of counts is two orders of magnitude higher for a given sample size and (ii) 0 s oxidises readily t o volatile OsOs, which has a boiling-point lower than most Re compounds, so that 0 s can be efficiently separated from 187Re. The importance of 0 s loss during sample preparation was minimised by combining the sample powder with a powdered Re -0 s isotopic spike prior t o any chemical treatment. The spike is in a nickel sulphide matrix and was made by a thioacetamide co-precipitation of Os, Re and Ni followed by a fire assay. The 1900s/'9*0s ratio in this preparation is 51.5 k 0.9, the 187Os/188Os ratio is 0.063 2 0.006 and the 185Re/187Re ratio is 17.04 2 0.41. These values are within the error limits quoted by the Oak Ridge National Laboratory and Techsnabexport, the suppliers of the metallic spikes. Parameters that significantly influence the 0 s analyses include the oxidising agent and the run temperature. The oxidising agent H5106 is preferred t o H202 or HNOB as it has the highest electrode potential, provides a prolonged, consistent reaction and is more stable when stored. Chilling the sample and the H5106, initially retards uncontrolled Os04 emission. Heat applied later in the run releases Os04 and helps maintain a high count rate. Thus, Os04 can be generated in a steady, controllable and reproducible manner.